This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/EP01/06326 which has an International filing date of Jun. 6, 2001, which designated the United States of America and which claims priority on German Patent Application number 100 37 379.8-32 filed Aug. 1, 2000, the entire contents of which are hereby incorporated herein by reference.
The present invention generally relates to a power converter, constructed using a multilevel circuit, for converting direct current to alternating or vice versa. The converter may include at least one capacitor and at least two semiconductor power switches. The or each capacitor may have a connection pair for connection of the capacitor to at least two semiconductor power switches.
Power converters using a multilevel circuit are known, for example, from U.S. Pat. No. 5,737,201. This document describes in particular the theoretical principles of a multilevel circuit. A multilevel circuit allows a power converter to be designed in a modular manner. Each of the modules includes at least two semiconductor power switches and at least one capacitor, which is arranged between the power switches. The special feature of the multilevel circuit is that the capacitors are not all of the same potential, but may be related to different potentials (so-called floating capacitors). An intermediate circuit voltage is passed to a number of floating capacitors in such a way that the voltage load on one semiconductor power switch is the result of the different between the voltage on two capacitors.
According to the prior art, the capacitors which are used in power converters have only one connection pair. The connection pair is connected to the semiconductor power switches in a first module. The semiconductor power switches in a further module are connected to the connection pair of the capacitor via an additional electrical connection. This additional electrical connection should be designed to have as low an inductance as possible, in order to reduce the load on the semiconductor power switches. U.S. Pat. No. 5,737,201 does not describe in any more detail the problem associated with the electrical connection between the capacitors and the semiconductor power switches having as low an inductance as possible.
An electrical connection with as low an inductance as possible can be achieved, in entirely general form, by special design measures. For example, a reduction in the size of an area through which a vertical component of a commutation circuit of a module flows leads to the electrical connection having a lower inductance. The inductance can be reduced considerably by using conductors that are as wide as possible between the capacitors and the semiconductor power switches, and by the distance between the forward conductor and the return conductor being as short as possible. A low-inductance electrical connection between the capacitors and the semiconductor power switches is subject to certain limits, relating to the design, accuracy and life of the power converter.
In order to allow a low-inductance electrical connection which can be physically implemented relatively easily between the capacitors and the power switches, it is known from EP 0 944 163 A1 for the capacitors in a power converter to be subdivided into two, and for one half of one capacitor to be associated with the semiconductor power switches of a first module of the power converter, and for the other half of the capacitor to be associated with the power switches of another module. Specific low-inductance electrical conductors, which are in the form of rail packs (so-called busbars) are used to connect the capacitor halves to the power switches of the associated modules. The rail packs have, for example, a laminated structure composed of two copper plates, which are used as conductors, with a plate or sheet composed of a solid insulator arranged between them. Owing to so-called partial discharges between the copper plates, the solid insulator is subject to aging, which restricts the life of the rail pack. Furthermore, low-inductance conductors in the form of rail packs cause problems in handling (mechanical loads on the rail packs can adversely affect their low inductance), and they are very expensive.
A further disadvantage of the power converter which is known from EP 0 944 163 A1 occurs with relatively low rating power converters, in which a single module with one capacitor and two power switches would be sufficient. Even in power converters such as these with only one capacitor, the capacitor is subdivided into two capacitor halves, which forces up the production costs for these known power converters.
One object of an embodiment of the present invention is to define and to develop a power converter of the type mentioned initially such that it is possible to produce an electrical connection between the capacitors and the power switches whose inductance is as low as possible, whose life is as long as possible, and which is as cost-effective as possible.
In order to achieve this object against the background of the power converter of the type mentioned initially, the an embodiment of the present invention proposes that the or each capacitor has at least one further connection pair for connection of the capacitor to at least two semiconductor power switches or to a DC network.
Thus, according to an embodiment of the present invention, at least one further connection pair is passed to the exterior on the or each capacitor. The capacitor can be connected via the one connection pair to at least two semiconductor power switches in a first module, and via each further connection pair to at least two further semiconductor power switches in further modules, or to a DC network. The connection pairs may be passed out of the capacitor at any desired point. However, it is recommended that the connection pairs be passed out where the design results in further modules of the power converter being arranged.
The capacitors which are used in power converters normally have two contact tracks, which run parallel to one another, in their interior, which contact tracks extend virtually over the entire capacitor length and between which a number of parallel-connected capacitor elements are arranged. Owing to the requirements for low inductance, these contact tracks may be in the form of low-inductance electrical conductors. Connection pairs can be passed out of the capacitor at virtually any desired points, originating from the contact tracks. Low-inductance electrical connections, which are generally provided in any case in capacitors for power converters, are thus used as low-inductance conductors for connection of the capacitors to the semiconductor power switches in the individual modules. Since there is no need for the relatively expensive rail packs (busbars) in the power converter according to the invention, the production and assembly costs of the power converter can be reduced to a major extent.
One advantageous development of an embodiment of the present invention proposes that the connection pairs of one capacitor be passed out of this capacitor on different sides of the capacitor. The capacitor may be physically arranged between the modules to whose semiconductor power switches it is intended to be connected. This allows the power converter to have a particularly compact construction.
One preferred embodiment of the present invention proposes that the or each capacitor have two connection pairs which are passed out of this capacitor on opposite sides of the capacitor. This allows the power converter to be formed from a number of modules arranged one above the other or alongside one another. This elongated structure of the power converter has the advantage that it is easy to see the individual components, so that assembly and maintenance are simplified. The elongated structure results in particular advantages when air is used to cool the semiconductor power switches, since the cooling airxe2x80x94in contrast to the situation in power converters whose construction is complex and angledxe2x80x94can be passed without any problems through the elongated power converter. Apart from being cooled by air, the power converter according to the invention may, of course, also be cooled in any other desired manner.
Each connection pair advantageously has two connecting contacts, each having at least one connecting element, with the connecting elements of each connecting contact being connected via a low-inductance flat rail conductor to a connection of a semiconductor power switch. In order to reduce the inductance of the connection between the capacitor and the power switches, one connecting contact of one connection pair may have a number of connecting elements running parallel to one another. The rail conductors are composed, for example, of copper and are insulated by air. An isolator panel may be arranged between the forward and return rail conductors, in order to avoid short circuits caused by foreign bodies and/or when air cooling is used, to carry the cooling air through the power converter, through its components. The isolator panel is composed, for example, of plastic.
In order to allow the cooling air to he routed in a particularly simple manner along the capacitors, when air cooling is used, another preferred embodiment of the invention proposes that at least one cold plate be arranged at a distance from at least one outer face of the or each capacitor. The distance between the outer faces of the capacitor and the cold plate may be used as a cooling channel, through which the cooling air can be passed laterally along the capacitor.
The upper face and lower face of the or each capacitor advantageously have areas which overhang the at least one outer face of the or each capacitor, and on which the at least one cold plate is mounted. This is of particularly simple structure for the cooling channels for the cooling air, but allows particularly effective air guidance.
The power converter is preferably in the form of an inverter, which converts a DC voltage to an AC voltage. Alternatively, the power converter according to the invention may also be in the form of a DC controller. The power converter according to the invention is particularly suitable for use in the medium-voltage and high-voltage ranges.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.